A multi channel photo detector (MCPD) is a widely used chroma measurement apparatus. A spectroscope of the MCPD can divide light passing through a sample by a full wavelength (380 nm-780 nm) using concave tortuous grids, and the MCPD can transform divided light with various wavelengths into a number of current signals using an optoelectronic array, calculate light spectrum according to the current signals, and finally display the calculated light spectrum.
In a measurement process using the MCPD, a measured sample is fixedly mounted on a platform, and the platform is moved so that the MCPD scans and measures the sample. For example, in the TFT-LCD industry, the MCPD is mainly used to measure red, green, and blue (hereinafter referred to as “RGB”) color resists of color filters. In order to measure the RGB color resists simultaneously, the RGB color resists should be horizontally arranged side by side. Since a width of a measuring light spot is generally about 40 μm, if sizes of the RGB color resists of the sample are small, an arranging direction of pixels of the sample should be perpendicular to a top side and a bottom side of a charge-coupled device (CCD) of the MCPD. Otherwise, the measuring light spot may be unable to entirely irradiate the color resists, the measurement data may be wrong, and judgment and evaluation of material properties of the sample may be adversely affected, which may result in that characteristics of products cannot meet requirements. As shown in FIG. 1 and FIG. 2, when such a colorimeter (i.e., an MCPD) is used to scan a conventional color filter 200, the color filer 200 is placed at a predetermined position. A width of a scanning light spot 300 of the colorimeter is generally about 40 μm and slightly less than widths of color resists 201, 202, 203 of the color filter 200. When the color filter 200 is placed correctly, as shown in FIG. 1, the colorimeter is driven according to predetermined step positions, the color resists 201, 202, 203 are irradiated by the scanning light spot 300 in turn, and the scanning light spot 300 does not irradiate any other portion of the color filter 200. However, if the color filter 200 is not placed at the predetermined position precisely, as shown in FIG. 2, and the colorimeter is still driven according to predetermined step positions, the scanning light spot 300 may partially irradiate out of the color resists 201, 202, 203 and result in measurement errors.